Unsaturated polyester networks are well known for their thermal and mechanical properties especially when reinforced with fibers and highly filled. Such materials are used in boat hulls and automobile body parts. These resins are inherently brittle, and attempts to toughen the compositions without sacrificing thermal and mechanical properties were made. U.S. Pat. No. 4,419,487 discloses the use of carboxyl terminated reactive liquid polymers to toughen polyester resins. U.S. Pat. No. 4,530,962 discloses that epoxy resin terminal groups on low molecular weight elastomers makes the molecules sufficiently compatible with polyester that a homogeneous dispersion of elastomer particles can be achieved on cure. This dispersion resulted in decreased brittleness and increases in fracture energy.